A known technique for reducing hydrocarbon emissions in internal combustion engines involves injecting secondary air into the engine exhaust manifold. This technique, usually referred to as Air Injection Reaction or simply AIR, is particularly important during engine warm-up following a cold start. Under such conditions, the air/fuel mixture supplied to the engine cylinders is relatively low (rich) to ensure stable combustion, and the injected air promotes rapid light-off of the exhaust system catalytic converter to enable early initiation of closed-loop fuel control. The secondary air promotes light-off of the catalytic converter by reacting with unburned hydrocarbons in the exhaust gas to produce heat, and by raising the air/fuel ratio in the exhaust system to an optimum level for converter light-off. In practice, however, it is difficult to optimize the injection of secondary air because the delivered airflow varies considerably depending on environmental factors such as ambient temperature and barometric pressure. This makes it difficult to take full advantage of AIR and can also lead to incorrect diagnosis of an AIR failure under conditions which severely restrict AIR airflow. Accordingly, what is needed in a method of accurately and reliably estimating AIR airflow, both for control and diagnostic purposes.